Ionic conductivity is usually associated with the flow of ions through an aqueous solution of metallic salts. In the vast majority of practical uses of ionic conductors, e.g., as electrolytes for dry cell batteries, the aqueous solution is immobilized in a paste or gelled matrix to overcome the difficulties associated with handling and packaging a liquid. However, even after immobilization, the system is still subject to possible leakage, has a limited shelf life due to drying out or crystallization of the salts and is suitable for use only within a limited temperature range corresponding to the liquid range of the electrolyte. In addition, the necessity of including a large volume of immobilizing material has hindered the aims of miniaturization.
In attempting to overcome the shortcomings of liquid systems, investigators have surveyed a large number of solid compounds hoping to find compounds which are solid at room temperature and have ionic conductances approaching those exhibited by the commonly used liquid systems. Such compounds have specific conductances at room temperature (20.degree. C.) in the range of 10.sup.-6 to 10.sup.-15 ohm.sup.-1 cm.sup.-1 as compared to aqueous solutions of salts which typically have a specific conductance of 0.5 to 0.05 ohm.sup.-1 cm.sup.-1.
Improved microelectronic circuit designs have generally decreased the current requirements for electronic devices. This in turn has enhanced the applicability of solid electrolyte power sources which usually can only deliver currents in the microampere range. These solid electrolyte systems have the inherent advantages of being free of electrolyte leakage and internal gassing problems due to the absence of a liquid phase and corrosion phenomena. In addition, they also have a much longer shelf life than the conventional liquid electrolyte power sources.
Gutman et al, J. Electrochem. Soc., 114, 323 (1967) discloses solid state cells utilizing cathodes of electronically conducting charge transfer complexes and anodes of selected divalent metals. U.S. Pat. No. 3,660,163 discloses solid state lithium-iodine primary cells employing a lithium anode, a solid state lithium halide electrolyte and a conductive cathode of organic materials, such as polycyclic aromatic compounds, organic polymers, heterocyclic nitrogen containing compounds, and the like, and iodine. U.S. Pat. No. 3,660,164 discloses solid state cells utilizing as a cathode a charge transfer complex in which the acceptor component is the halogen and the donor component is an organic compound, typically aromatic or heterocyclic.
Although various cathode materials have been recited in the art for use in various cell systems, an object of the present invention is to provide a novel cathode for use in solid electrolyte cell systems.
Another object of the present invention is to provide a cathode comprising a charge transfer complex in which the complex is the reaction product of at least one halogen with a carbonaceous pitch.
Another object of the present invention is to provide a cathode comprising a charge transfer complex in which the complex is the reaction product of iodine or bromine with mesophase pitch.
Another object of the present invention is to provide a cathode comprising a charge transfer complex in which the complex is the reaction product of iodine with mesophase pitch for use in a solid state cell employing a lithium anode and a solid lithium iodide electrolyte.
The foregoing and additional objects will become more fully apparent from the following description.